Transparent Conductive Oxides for Organic Electronics

By

Graham Blair Murdoch

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Graduate Department of Materials Science and Engineering University of Toronto

© Copyright by Graham Blair Murdoch (2010)

ii

Alternative Transparent Conductors for Organic Electronics

M.A.Sc. Thesis 2010

Graham Blair Murdoch

Graduate Department of Material Science and Engineering

University of Toronto

Abstract

Organic solar cells and organic light emitting diodes are on the forefront of

emerging technologies aimed at harnessing light in ways never thought possible. Large-

area installations of OLED solid state lighting (SSL), as well as organic photovoltaics

(OPVs), will become possible as the efficiencies of these devices continue to rise. All

organic solar cells and OLEDs require the use of transparent conductive electrodes.

Indium oxide (ITO) is currently the transparent conductor of choice for these

applications, due to its unique combination of transparency, high conductivity, durability,

and favourable surface properties.

Indium, however, is a rare and expensive metal; proposed large-area installations

of OPV cells and OLEDs will add further strain to global indium supply. Transparent

conductive materials that are abundant, inexpensive, and which enable efficient and

robust organic devices must therefore be developed. In the present work, suitable ITO

anode replacement materials are demonstrated for OLEDS, small-molecule, polymer, and

PbS colloidal quantum dot photovoltaics.

iii

Acknowledgements

Throughout the course of the last two years, Ihave had the privilege of working

alongside many exceptionally bright, diligent, and gracious individuals. I consider these

people my colleagues and also my friends. Without their help and guidance the present

work would certainly not have come to fruition, and I would have been deprived of all

that they have taught me during our time together.

First and foremost, I would like to thank and acknowledge Dr. Zhenghong Lu,

who was an excellent teacher and mentor. Dr. Lu tried his utmost to foster an

environment of free creative expression while providing direction and knowledge

necessary for success, and for these I am truly grateful.

I would also like to acknowledge my labmates, Leo Mordoukhovski, Mark

Greiner, Michael Helander, Zhibin Wong, Dong Gao, Fanjie Wei, and Sai Wing Tsang,

all truly outstanding individuals who were always there to lend a hand and to offer their

insights.

iv

Table of Contents

Abstract

Acknowledgements

Table of Contents

List of Tables and Figures

Chapter 1. Introduction and Motivation......................................................................01

1.1 Indium Tin Oxide.......................................................................................01 1.2 Indium Tin Oxide Alternatives..................................................................04

Chapter 2. Overview of organic photovoltaics.............................................................09

2.1 Organic Photovoltaics Defined..................................................................09

2.2 Why Organic Photovoltaics.......................................................................10

2.2.1 Photovoltaic Technology...............................................................10

2.2.2 The Need for Renewable Energy...................................................11

2.2.3 Renewable Energy Alternatives—Organic PV and Grid Parity....14

2.3 Device Physics of Organic Solar Cells......................................................18

2.4 Raising Efficiencies and Lowering Costs..................................................22

2.4.1 The ‘Exciton Bottleneck’..................................................................22

2.4.2 Contacts—Efficient ITO Replacement ............................................24

Chapter 3. A Comparison of CuO and Cu2O hole-injection layers for low voltage organic devices.............................................................................................27

3.1 Introduction...............................................................................................27

3.2 Experimental..............................................................................................28

3.3 Results and Discussion..............................................................................29

3.4 Conclusion.................................................................................................36

Chapter 4. Aluminum doped zinc oxide for organic photovoltaics............................37

4.1 Introduction...............................................................................................37

4.2 Experimental..............................................................................................38

v

4.3 Results and Discussion..............................................................................40

4.4 Conclusion.................................................................................................48

Chapter 5. Transparent Conductive Oxides for Organic Photovoltaics...................49

5.1 Introduction...............................................................................................49

5.2 Experimental..............................................................................................51

5.3 Results and Discussion..............................................................................52

5.4 Conclusion.................................................................................................58

Chapter 6. FTO for PbS Colloidal Quantum Dot Photovoltaics.................................60

6.1 Introduction...............................................................................................60

6.2 Experimental..............................................................................................62

6.3 Results and Discussion..............................................................................64

6.4 Conclusion.................................................................................................66

Chapter 7. Summary.......................................................................................................67

Chapter 8. References.....................................................................................................71

vi

List of Tables and Figures

Figure 1.1 Band structure of tin doped indium oxide (ITO). Valence and conduction bands arise from O 2p and In 5S atomic orbitals. The wide bandgap of indium oxide, combined with degenerate doping of the In 5s conduction band provides high transparency and metallic conductivity [2]. ................1

Figure 1.2. Unit price of Indium in 1998 dollars per ton from 1987 to 2007 [4]. .........2

Figure 1.3. Worldwide indium production from 2000 to 2008 [4]. ............................. 3

Table 1.1. Carrier concentrations and minimum resistivities achieved for ZnO doped with various impurity elements. To date, aluminum doped ZnO (AZO) and gallium doped ZnO (GZO) have exhibited the highest conductivities of this group [17]. ............................................................................................6

Table 1.2. Summary of binary, ternary, and multicomponent transparent oxide

systems showing metallic conductivity. .....................................................7 Figure 2.1 Device structure of a typical organic photovoltaic cell, comprising an

organic stack, a metallic cathode, and a transparent conductive anode. ….9

Table 2.1. Classification of Photovoltaic Technologies. …………………………...10

Figure 2.2. Percentage composition of Canada’s energy generation capacity as of (2006) [6]. ……………………………………………………………….12

Table 2.2. Global renewable energy generation capacity by type. …………………15

Figure 2.3. U.S. landmass required to provide 3.3 TW of solar electricity assuming 200W/m2 average insolation and 10% cell efficiency [8]. ……………...16

Figure 2.4. Historical photovoltaic price and generation capacity extrapolated to 2023

[9]. .............................................................................................................17 Figure 2.5. Continued gains in power conversion efficiency for small molecule and

polymer solar cells from 1975 to the present day. Maximum achieved efficiencies for amorphous silicon devices are shown for reference. …...18

Figure 2.6. Depiction of electron-hole pair generation on copper phthalocyanine, a

planar organic dye molecule used in organic solar cells. ………………..19

Figure 2.7. Energy level positions of free and bound electron-hole pairs in organic molecules. The electrostatic force binds electrons and holes together into an exciton, lowering the energy of the electron-hole pair. ……………...20

vii

Figure 2.8. Stepwise operation of donor acceptor organic photovoltaic devices: a) absorption, b) relaxation (exciton formation), c) exciton diffusion to donor acceptor interface, d) charge transfer and free carrier generation, e) charge collection at electrodes [13]. …………………………………………….22

Figure 2.9. Tandem small molecule organic solar cell. Two subcells provide two donor acceptor interfaces for improved light harvesting. The resultant open circuit voltage is the summation of the open circuit voltages of the subcells [15]. …………………………………………………………….23

Figure 2.10. Equivalent circuit representation of an organic photovoltaic device. For efficient device operation, a small equivalent series resistance and a large equivalent shunt resistance is desirable. ………………………………...24

Figure 2.11. J-V characteristics for copper phthalocyanine based organic solar cells fabricated on GZO and ITO anodes. Cells fabricated on GZO show increase series cell resistance due to the existence of an energetic barrier to hole extraction [18]. ……………………………………………………..25

Figure 3.1. Current density vs. voltage and luminance vs. voltage for OLEDs made with UV-oxidized Cu and ITO anodes. ....................................................30 Figure 3.2. a) Cu2p3/2 XPS and b) CuLMM Auger spectra (hν=1486.6 eV) for 5 nm

Cu films heated at 100 °C for 60 minutes and UV-ozone treated for 60 minutes. ....................................................................................................31

Figure 3.3. a) Current density vs. voltage for hole-only structures as follows: Anode/NPB (500nm)/Cu. The anodes used were Cu/CuO and Cu/Cu2O. Fits to the Scott and Malliaras injection model are included as solid lines, along with

bφ values, see Ref. 10 and 11. b) Current density vs. voltage and luminance vs. voltage (inset) for OLEDs made with CuO and Cu2O capped ITO anodes. ..................................................................................33

Figure 3.4. He I UPS valence band spectra for a) CuO and b) Cu2O substrates with

sequential NPB deposition up to a thickness of 12.8 nm. Work function shifts and C1s core level shifts vs. NPB thickness are plotted for c) CuO substrate and d) Cu2O substrate. ...............................................................35

Figure 4.1. a) Optical Transmission vs. Wavelenth for untreated, DC sputtered AZO

films deposited at 8, 12, 16 and 19 cm target-to-substrate distances. b) Film resistivity for as-deposited and ozone treated AZO vs. target-to-substrate distance. .....................................................................................41

Figure 4.2. SEM micrographs of AZO films grown at target-to-substrate distances of

a) 19 cm, b) 16 cm, c) 8 cm. .....................................................................43

viii

Figure 4.3. a) Current density and b) Photocurrent density vs. voltage for OPV devices made using as-deposited and ozone treated AZO. Current densities were measured under 100 mW/cm2 and dark conditions. Photocurrent density at a specified voltage was calculated as the difference between device current densities measured under illuminated and dark conditions. .................................................................................................44

Figure. 4.4. Current density at 100 mW/cm2 and dark illumination levels vs. voltage

for OPV cells made using ozone treated AZO as well as ITO. ................47 Table 4.1. Open circuit voltage VOC, short circuit current density JSC, fill factor

FF, Power Conversion Efficiency ηP, and the calculated series resistance RS of all cells. ............................................................................................48

Figure 5.1. Secondary cutoffs (a) and core level shifts (b) for ozone treated and

untreated AZO and FTO. ..........................................................................53 Table 5.1. Open circuit voltage VOC, short circuit current density JSC, fill factor FF,

Power Conversion Efficiency ηP, calculated series resistance RS, and shunt resistance Rshunt of cells fabricated on various transparent conductive anodes. ......................................................................................................54

Figure 5.2. a) JV curves for PCBM:P3HT solar cells under 100 mW/cm2 AM1.5G

illumination and dark (inset). b) JV curves for PCBM:P3HT devices fabricated on untreated and ozone treated FTO and AZO under 100 mW/cm2 AM1.5G illumination. ...............................................................55

Figure 5.3. a) Schematic band alignment of a bulk heterojunction device under the

condition VOC < Vbi. b) Band alignment of a BHJ device with an electron blocking anode under the condition VOC > Vbi. ........................................57

Figure 6.1. Photovoltaic cells (a and b) and hole only devices (c and d) employing

FTO and ITO anode materials. Device structures are as follows: a) ITO/PbS/Al, b) FTO/PbS/Al, c) ITO/PbS/Au, d) FTO/PbS/Au. ..............63

Figure 6.2. AFM image of a) FTO and b) ITO surfaces. The surface of unpolished FTO is much rougher than that of polished ITO. ......................................64

Figure 6.3. Current at 80 mW/cm2 and dark illumination levels vs. voltage for CQD cells made using ozone treated ITO (device a) and FTO (device b). ..............................................................................................................65

Figure 6.4. Current at 80 mW/cm2 and dark illumination levels vs. voltage

for CQD hole-only devices made using ozone treated ITO (device c) and FTO (device d). .........................................................................................66

1

Chapter 1. Introduction and Motivation

1.1 Indium Tin Oxide

Conventional organic photovoltaics (OPVs) and organic light emitting diodes

(OLEDs) are generally fabricated on indium tin oxide (ITO) substrates [1]. Indium tin

oxide is a transparent conductive oxide that has recently found widespread use in flat

panel displays and televisions. ITO is unique in its combination of optical transparency,

metallic conductivity, and high work function, making it an excellent contact to p-like

organic molecules. ITO is a degenerately doped n-type semiconducting oxide, with

bandgap of approximately 3.75 eV [2]. Figure 1.1 shows the band structure of ITO. The

wide bandgap formed between the O 2p6 states and In 5s states permits high transparency,

while the degenerate doping of the In 5s band by Sn3+ donor states introduces a high

density of free electrons.

Figure 1.1 Band structure of tin doped indium oxide (ITO). Valence and conduction bands arise from O 2p and In 5S atomic orbitals. The wide bandgap of indium oxide,

2

combined with degenerate doping of the In 5s conduction band provides high transparency and metallic conductivity. [2]

The widespread use of ITO in optoelectronic devices has recently become

problematic. Surging market demand for indium base metal for use in flat panel displays,

combined with limited global reserves of recoverable indium, has led to significant price

increases in recent years. The cost of indium tin oxide in 2006 reached more than ten

times its price in 2003 [3]. Figure 1.2 shows the price of Indium per ton adjusted for

inflation to 1998 dollars [4]. The unit price of indium has historically fluctuated with the

business cycle, showing highs and lows approximately every 8 years. The most recent

high occurred in 2005, with indium reaching prices of 789,518 USD/t. Taking into

account historic price variations, the price rise in 2005 was uncharacteristically large,

attributable to sudden demand for flat panel display and televisions production.

Figure 1.2. Unit price of Indium in 1998 dollars per ton from 1987 to 2007 [4].

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

800,000

900,000

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

Price (98$

/t)

Indium Price

3

Since the year 2000, global indium usage has risen exponentially. Indium

production currently totals approximately 568 t per year. Refinery production of indium

base metal from 2000 onward is given in figure 1.3. [4] USGS data indicate a global

economically recoverable indium reserves base of 16,000t based on concentrations of In

in zinc ores [5]. Although estimates have varied due to the inherent difficulty in

ascertaining the reserves base [6], an acute pinch in the supply of Indium will almost

certainly be felt in the next half century unless suitable alternatives to ITO are adopted;

the proposed addition of large-area solar installations, as well the development OLED-

based solid state lighting installations will add entire industrial sectors requiring heavy

usage of indium. A one gigawatt copper indium gallium diselenide (CIGS) installation,

for instance, requires approximately 50 tons of indium [5]. The demand for indium is

therefore expected to grow unabated, necessitating the need for acceptable alternatives to

ITO.

Figure 1.3. Worldwide indium production from 2000 to 2008 [4].

300

350

400

450

500

550

600

2000 2001 2002 2003 2004 2005 2006 2007 2008

Refin

ery Prod

uction

(t)

Year

Indium Refinery Production

4

1.2 Indium Tin Oxide Alternatives

Suitable alternatives to indium tin oxide for use in organic solar and OLED devices

must offer the possibility of equivalent or improved device performance as well as cost

savings when compared to devices currently made with ITO. Alternatives will have the

following characteristics:

• Highly transparent

• Highly conductive

• Suitable surface morphology

• Resistant to chemical attack, thermal, and plasma environments

• Nontoxic

• Durable films

• Low series resistance on contact with organic films

• Material compatibility (wetting, interdiffusion)

• Low cost

• High deposition rate

Over the past 30 years, a number of alternative transparent conductive oxide (TCO)

materials have been discovered. Most highly conductive oxides discovered to date have

been n-type oxides, although recent work has been done on high-mobility p-type oxides

such as NiO [7], CuAlO2 [8], CuGaO2 [9], SrCu2O2 [10], and LaCuOS [11]. The

conductivity of these p-type oxides is not currently sufficient to enable their use as ITO

replacements. P-type oxides have, however, found widespread use as buffer layers in

various organic devices.

5

When deposited as ultrathin films (<10 nm), p-type oxides have been found to

improve hole injection into OLED devices, raising device power efficiency by lowering

the driving voltage required to inject carriers. Typically, a p-type oxide such as MoO3

[12] or NiO [13] is deposited as a buffer layer onto an ITO substrate prior to depositing

the organic layers. P-type oxide buffer layers have also found use in OPV cells.

Recently, a buffer layer of NiO deposited on an ITO anode was shown to dramatically

increase the performance of polymer solar cells. [14] This performance increase was

attributed to the ability of the p-type NiO to extract holes from the polymer layer, while

blocking undesired leakage of electrons at the anode contact.

Chapter 3 investigates the use of p-type Cu2O/Cu and CuO/Cu bilayers as ITO-

replacement anodes for OLED devices. In this work, ultrathin films of copper are

surface-oxidized using thermal and ozone means to produce bilayers of conductive

copper and copper(I)/copper(II) oxides. Owing to the conductivity of the copper

underlayer in both cases, the films could be made to thicknesses of <15 nm while still

retaining sheet resistances similar to those of ITO films. At these thicknesses, the copper

layers were semitransparent, allowing adequate light to escape the OLED devices. With

device efficiency comparable to identical devices produced with ITO, the use of oxidized

copper layers deposited by simple evaporation offers a low-cost indium-free route for

OLED production.

Most commercially used TCOs are n-type thin films of impurity doped binary

compounds of SnO2 or In2O3 deposited by chemical or physical deposition techniques.

6

Most recently, zinc oxide has been developed as an alternative TCO material. The

discovery of the usefulness of ZnO as a highly conductive transparent material was made

in the early 80s [15]. Although undoped films of pure ZnO are quite conductive (4.5x10-4

Ωcm), they were found to be thermally unstable beyond 150 °C. Later, the addition of Al

as an impurity was discovered to effectively raise the thermal stability to 400 °C, and

lower the resistivity to 1.9 x 10-4 Ωcm [16]. Doping ZnO with various group III elements

such as Al, Ga, In, B or group IV elements such as Si, Ge, Ti, Zr and Hf has since been

investigated. Table 1.1 shows the resistivity and carrier densities achieved for ZnO doped

with various impurities [17]. Aluminum and gallium doped zinc oxides have generally

shown the lowest resistivities.

Table 1.1. Carrier concentrations and minimum resistivities achieved for ZnO doped with various impurity elements. To date, aluminum doped ZnO (AZO) and gallium doped ZnO (GZO) have exhibited the highest conductivities of this group [17].

Dopant Doping Content

(at.%) Resistivity (10-4 Ωcm)

Carrier Concentration (1020 cm-3 )

Al 1.6-3.2 1.3 15.0 Ga 1.7-6.1 1.2 14.5 B 4.6 2.0 5.4 Y 2.2 7.9 5.8 In 1.2 8.1 3.9 Sc 2.5 3.1 6.7 Si 8.0 4.8 8.8 Ge 1.6 7.4 8.8 Ti 2.0 5.6 6.2 Zr 5.4 5.2 5.5 Hf 4.1 5.5 3.5 F 0.5 4.0 5.0

7

Ternary compounds made from combinations of SnO2, In2O3, CdO and ZnO have

also been developed in recent years. Typically, these systems exhibit composition-

dependant conductivity similar to metallic alloy systems and composition-dependent

optical properties similar to mixed films of dielectric materials [17]. Although the use of

multicomponent oxides enables rational design of TCO films in terms of their various

electrical, optical, chemical and mechanical properties, these films are not widely in use

due to the difficulties associated with controlled deposition of multicomponent systems.

Table 1.2. Summary of binary, ternary, and multicomponent transparent oxide systems showing metallic conductivity.

TCO System Dopants SnO2 Sb, F, As, Nb, Ta In2O3 Sn, Ge, Mo, F, Ti, Zr, Hf, Nb, Ta, W, Te ZnO Al, Ga, B, In, Y, Sc, F, V, Si, Ge, Ti, Zr, Hf CdO In, Sn ZnO-SnO2 ZnO-In2O3 In2O3-SnO2 CdO-SnO2 CdO-In2O3 ZnO-In2O3-SnO2 CdO-In2O3-SnO2 ZnO-CdO-In2O3-SnO2 MgIn2O4 GaInO3 Sn, Ge CdSb2O6 Y

Despite the fact that TCO materials have been studied for over 40 years, the only

materials in practical use are SnO2, In2O3, ZnO, ternary compounds, multicomponent

oxides, and doped versions thereof. Table 1.2 summarizes the available transparent

oxides exhibiting metallic conductivity. Notably, after one eliminates any oxides

containing In and Cd for the purposes of cost reduction and non-toxicity, one is left with

8

impurity doped tin oxides and zinc oxides as remaining practical options. Surprisingly,

very little investigation has been carried out on the usage of indium-free TCOs for solar

applications. Chapters 4, 5 and 6 are devoted to enabling the usage of indium-free TCOs

for small-molecule, polymer, and colloidal quantum dot solar cells.

9

Chapter 2: Overview of Organic Photovoltaics

2.1 Organic Photovoltaics Defined

Organic solar cells are photovoltaic cells comprised, in whole or in part, of one or

more polymer or small molecule organic compound. Cells typically also comprise other

thin film inorganic layers, such as metal or oxide layers or phases. An example of the

latter case may include cells having regions in which inorganic nanocystals are embedded

into a polymer matrix [1]. Typically, the device structure of an organic photovoltaic cell

is a layered stack that includes two electrodes necessary to extract photogenerated charge,

and one or more organic layers located between the electrodes. Figure 2.1 depicts this

typical structure. As will be discussed, the physics of organic photovoltaic cells differs

significantly from traditional crystalline and other thin film PV devices.

Figure 2.1 Device structure of a typical organic photovoltaic cell, comprising an organic stack, a metallic cathode, and a transparent conductive anode.

10

2.2 Why Organic Photovoltaics?

2.2.1 Photovoltaic Technology

Organic solar cells fall into the general category of ‘third-generation’ photovoltaic

technology. Table 2.1 shows a high level breakdown of the various categories of existing

solar technologies.

Table 2.1. Classification of Photovoltaic Technologies

I Generation II Generation III Generation • single crystal wafers • single pn junction • expensive

production • max theoretical

efficiency 31%

• Thin film deposition • Eg. CIGS, α-Si,

CdTe/CdSe • Sputtering, PVD,

CVD, PECVD, electroplating, etc.

• 19.9% maximum achieved efficiency [4]

• Moderate efficiency/ ultra low cost

• Ultra high efficiency/moderate cost

• Inexpensive materials

• Low temperature deposition

• High speed roll-to-roll processing

First-generation PV devices are based on single-crystalline silicon wafers. In

these devices, a single PN junction is used to define the device photoactive region. Given

the properties of single-crystalline silicon, and the sun’s power spectrum, the maximum

theoretical efficiency for this type of device is approximately 30% (Shockley Quiesser

limit) [2]. The highest reported practical efficiency for a single junction silicon cell is

24.7%. [3]

Second generation solar cells are, in general, devices made without the need for

single-crystal wafers. Notable examples of this type of technology are CIGS, α-Si, and

11

CdTe/CdSe based cells. Second generation solar cells use well established techniques of

thin film processing such chemical vapor deposition (CVD), sputtering, ultrasonic

nozzles, electroplating, etc. These types of cells attempt to address the need for large-area

solar production while mitigating the high energy and production costs normally

associated with first generation devices. The highest reported power conversion

efficiency for a second generation device, 19.9%, was achieved by NREL using a

surface-modified CIGS architecture. [4]

Third generation solar cells, to which OPV devices belong, are a further category

that offers the promise of greatly reduced cost per watt of generated power. This category

encompasses ‘future’ technology which will greatly enhance efficiency and/or drastically

reduce manufacturing costs. Organic solar cells are classified as third generation

technology by the latter criterion. Owing to their ability to be easily processed, organic

materials are desirable for their inherent potential to reduce overall device costs. The

ability of organic materials to be spin coated, spray-coated, and even ink-jet printed on a

roll-to-roll basis can potentially obviate the need for enormously expensive

semiconductor fabrication facilities. Furthermore, simple coating processes can eliminate

hazardous substances involved in traditional semiconductor and thin film processes.

2.2.2 The Need for Renewable Energy

Total global energy consumption is currently estimated to be 15.8 TW [5].

Electrical and mechanical energy production in Canada and the United States is derived

predominantly from petroleum, natural gas, coal, nuclear, and hydro, with a very small

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coal will be depleted in 137 years, and natural gas in 63 years [6]. As the recovery of

dwindling coal, petroleum, and natural gas resources becomes progressively more

difficult, energy costs will rise. This is also to say nothing of the environmental toll

associated with retaining an energy mix dominated by fossil fuels.

In order for sustainable energy to enter and succeed in the marketplace, it must be

as inexpensive and easy to employ as grid power. This concept is called ‘grid parity’.

Without grid parity, interventions such as legislation to forbid the use of (currently)

cheaper non-renewable fossil sources, or the introduction of carbon taxes, are necessary

in order to give renewable energy sources an artificial competitive advantage. I remain

optimistic, however, that the imposition of legislation and taxes will not be necessary

through the efforts of many scientists, engineers and entrepreneurs around the world to

create truly competitive sources of energy.

In any foreseeable scenario, the price of energy derived from fossil sources will

inexorably rise due to diminishing resources. If cost-competitiveness currently limits the

implementation of renewable sources, this barrier will inevitably crumble. The time

required, however, to reach this equivalency is the central issue when environmental

concerns are factored in. The following questions arise: Will planetary environmental

disaster related to the usage of fossil fuels (vis à vis global warming), occur on a time

scale faster than the inevitable market changeover to renewable sources, if at all?

Secondly: Can government intervention practically accelerate the adoption of a

renewable energy mix in the required time frame? If the answer to the first question is no,

14

then we need not worry. If the answer to the first question is yes, and the answer to the

second question is no, then we have a problem! Assuming, of course, that we are not

already past the ‘point of no return’ in terms of planetary disaster (and I am optimistic

that we are not), the best way to proceed is to focus on technology. The development of

cheap and sustainable energy—making the answer to the first question a resounding

‘yes’—is critical to securing our energy future.

2.2.3 Renewable Energy Alternatives—Organic PV and Grid Parity

Discounting solar generation for a moment, the renewable energy options which

remain are, in total, insufficient to meet global demand. These options are Hydro, Wind,

Biomass, and Geothermal. Hydroelectricity is both an established and competitive

generation technology. The problem with hydro, however, exists in the fact that there are

simply not enough suitable locations on earth for hydroelectric plants to make a

significant contribution to the global energy mix. Similar practical limitations exist for

wind, biomass, and geothermal energy sources. Table 2.2 outlines the practical limits for

energy generation in each category [7]. We see that the sum total of feasible generation

capacities in each category falls below the globally required 16+ TW. Therefore,

although the aforesaid options will each occupy a certain definite share in the energy

market of the future, we will most certainly require solar energy as a critical component

of earth’s future energy mix.

15

Table 2.2. Global renewable energy generation capacity by type [7].

Type Capacity Hydro • 1.5 TW Technologically feasible

• 0.9 TW economically Wind • Practical limit: 2 TW using 4% of Class >3 Land Biomass • 0.3% efficient

• water Limited • 20 TW would require 31% land use • Practical limit 5-7 TW.

Geothermal • Total continental geothermal potential: 11.6 TW • Total oceanic geothermal potential: 20 TW • mostly unfeasible:

cost per meter of drill depth rise exponentially

wells last 5 years on average before redrilling

Solar energy has the potential to fill the gap left by other renewable sources. The

amount of solar energy flux impinging on the Earth’s surface is approximately 10,000

times worldwide energy consumption. As an example, at 10% solar cell power

conversion efficiency and daily average flux of 200 W/m2 (accounting for night/cloud

cover), 1.7% of the U.S. landmass covered with photovoltaic cells could completely

power the most energy hungry nation on earth [7]. Figure 2.3 shows the approximate

required area as a fraction of the landmass continental United States.

16

Figure 2.3. U.S. landmass required to provide 3.3 TW of solar electricity assuming 200W/m2 average insolation and 10% cell efficiency. [7]

The relevant parameter determining the economic viability of solar cell

technology is the cost per Watt of peak generation capacity, calculated as:

2

2

[$ / ]$ /1000 /P

Cost mWPeak Efficiency W m

=×

Government incentives and carbon taxes notwithstanding, the grid parity cost of energy is

approximately 0.3 $/WP. Figure 2.4 shows the learning curve for solar cell technology

from 1976, extrapolated to 2023 [8]. By 2023, the cost of first-generation crystalline Si

devices is expected to level out just above 1 $/WP. Similarly, thin film devices are

17

expected to level out at approximately 0.5 $/WP. New technologies, namely third

generation devices including organic solar cells, are expected to take the cost down to

grid parity levels, meeting the goal of the U.S. Department of Energy.

Figure 2.4. Historical photovoltaic price and generation capacity extrapolated to 2023 [8].

The central challenge for organic photovoltaic devices is to raise efficiencies. An

approximate upper bound for organic power conversion efficiency was calculated by

Stephen R. Forrest to be approximately 20% [9]. Figure 2.5 tracks the progress of

achieved cell efficiencies over three decades. Both polymer- and small molecule-based

devices have rivaled each other in efficiency, currently reaching values above 5%. Given

the low costs of producing OPV devices, achievement of 15% PCE will meet and exceed

the D.O.E. goal of 0.3 $/WP.

18

Figure 2.5. Continued gains in power conversion efficiency for small molecule and polymer solar cells from 1975 to the present day. Maximum achieved efficiencies for amorphous silicon devices are shown for reference.

2.3 Device Physics of Organic Solar Cells

Organic photovoltaics are unlike traditional single-crystal photovoltaics in a

number of respects. In the following section, the basic operation of OPVs will be

discussed.

In single-crystalline semiconductor devices, a PN junction is used to create a

depletion region with an associated electric field. When photons with energy greater than

the bandgap energy of the semiconductor are incident on the PN junction, electron-hole

pairs are generated. The built in electric field subsequently sweeps the carriers out of the

depletion region, whereupon they exit the crystal to flow through an external load.

An organic solar cell, on the other hand, uses small molecule or polymer

substances. Figure 2.6 depicts a small-molecule OPV device comprising a photoactive

organic lay

commonly

Photons inc

individual m

Figure 2.6.

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19

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21

[11]. In order to adequately furnish the required electric fields to unbind the electrons

and holes, and thus to achieve high device quantum efficiency, donor/acceptor organic

systems have been adopted. In these bilayer devices, an electron accepting organic layer

with high electron affinity energy is used in tandem with a donor molecule of lower

electron affinity. In 1986, tang introduced the first donor acceptor device with power

conversion efficiency of a record 1% [12].

Figure 2.8 depicts the stepwise operation of a typical donor-acceptor device.

Incident photons are first absorbed in an ultrafast process, typically requiring only 10-15 to

10-16 seconds, producing an electron-hole pair. The optical absorption length of organic

absorbers such as the aforementioned copper phthalocyanine is typically ~100 nm.

Immediately, the electron and hole relax in energy as they mutually become bound into

an exciton. This process requires between 10-9 to 10-13 seconds. Having been generated,

the excitons in the absorber layer diffuse through the layer. The lifetime for this diffusion

before recombination occurs is ~10-9 seconds. The corresponding diffusion length before

recombination is between 5nm to 20 nm. Excitons that successfully diffuse to the donor-

acceptor interface become unbound as the electron is accepted by the organic acceptor

material. The charge transfer process is ultrafast, requiring only ~10-13 seconds.

Subsequent to charge separation, free holes and electrons migrate away from the donor

acceptor interface and exit through the anode and cathode contacts. [13]

Figure 2.8.

2.4 Rais

2.4.1 Th

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23

one photoactive interface is used in series, resulting in a net open circuit voltage that is

the summation of the open circuit voltages of the subcells. Figure 2.9 shows the device

structure of one such device produced by Forrest et al., yielding a power conversion

efficiency of 5.7%. [14]

Figure 2.9. Tandem small molecule organic solar cell. Two subcells provide two donor acceptor interfaces for improved light harvesting. The resultant open circuit voltage is the summation of the open circuit voltages of the subcells [14].

Bulk heterojunction devices are also an attempt to raise the effective donor

acceptor interface area to capture more of the available light. While this strategy has been

effective in raising the active area, one runs into problems of lower carrier mobility as the

carriers must wend their way through a film of complex phase structure, occasionally

encountering an insurmountable cul-de-sac. Further strategies to overcome the exciton

bottleneck include nanostructured interfaces to increase the donor-acceptor interface area

[15], or the

acceptor int

2.4.2 Co

Bef

and electro

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extraction e

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Figure 2.10

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24

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25

Figure 2.11 shows the results of a recent attempt to replace ITO for Gallium

doped ZnO (GZO) as the transparent anode material in a CuPc/C60 device [17]. When

compared to the device fabricated on ITO, the GZO device exhibited a much larger series

resistance. Large series resistance manifests in a reduced slope at the open circuit (zero

current) condition. Despite the increased series resistance for the GZO device, the short

circuit currents for both devices remained comparable, indicating an equivalent number

of carriers exiting the organic system at the contacts. The above attempt to replace ITO

with GZO was unsuccessful due to the contact resistance at the GZO anode, lowering

device power conversion efficiency.

Figure 2.11. J-V characteristics for copper phthalocyanine based organic solar cells fabricated on GZO and ITO anodes. Cells fabricated on GZO show increase series cell resistance due to the existence of an energetic barrier to hole extraction [17].

26

In addition to lowering the series resistance at the electrode contacts, it is

desirable to raise device shunt resistance. Low shunt resistances arise when holes and

electrons are offered an alternative path to recombine without first traversing the external

circuit. If holes and electrons both tend to exit the anode or cathode at the same time, they

will recombine, reducing the photocurrent. In this case, one will observe reduced short

circuit current, and a smaller shunt resistance, indicated by a larger slope at the short

circuit (zero bias) condition. Holes and electrons are produced together in the organic

layers when excitons are unbound at donor acceptor interfaces. In bilayer donor acceptor

devices, such as the CuPc/C60 device shown above, free holes tend to be localized on the

donor side of the device, and free electrons tend to be localized on the acceptor side of

the device. Because of this geometry, the ability of the electrodes to exclusively extract

only holes or electrons is of less consequence to the device shunt resistance. For bulk

heterojunction devices, however, holes and electrons occupy the same geometrical

volume because donor and acceptor materials are blended together. It is therefore

important that an anode contact to such a blend be capable of accepting holes and

rejecting electrons, and vice versa for the cathode contact.

The work which follows in chapters 4 and 5 center on finding alternative

transparent anode contacts that produce equivalent, or improved device performance for

both planar and bulk heterojunction devices.

27

Chapter 3. A Comparison of CuO and Cu2O hole-injection layers for low voltage organic devices

3.1 Introduction

Cu2O and CuO have been grown with an aim to reduce junction electrical

resistance when interfaced with N,N’-bis (1-naphthyl)-N,N’-diphenyl-1,1’ biphenyl 4,4’-

diamine (NPB). Organic light-emitting diodes (OLEDs) employing Cu/CuO anodes have

equivalent driving-voltages as devices made with indium-tin-oxide (ITO). Hole-injection

barriers are calculated from current-voltage characteristics of CuO/NPB/Cu and

Cu2O/NPB/Cu devices via theoretical simulation. Photoelectron spectroscopies are used

to measure oxide valence band spectra, interfacial dipole formation, and band bending

during in situ sequential deposition of NPB on each oxide. Calculated hole-injection

barriers and those derived from photoemission results accord well, explaining superior

hole-injection at the CuO-NPB interface.

With steady adoption for niche markets such as flat-panel displays, organic

electronics are now actively researched for other applications such as organic solid-state

lighting, thin film transistors, memory units, photovoltaics, etc. Indium tin oxide (ITO)

has been used ubiquitously as an anode material, despite its ever-rising cost and technical

challenges in current-driven devices such as organic light-emitting diodes (OLEDs).

Recently, the use of metal oxides as hole-injection layers (HILs) for anodes has attracted

considerable interest1,2,3,4. In particular, the Cu/CuxO system has shown promise as an

efficient anode contact for OLEDs,5,6 organic thin film transistors (OTFTs)7 and

photovoltaics8. In this work, we report that OLEDs based on N,N’-bis (1-naphthyl)-

N,N’-diphenyl-1,1’ biphenyl 4,4’-diamine (NPB) employing ultraviolet (UV) -ozone

28

produced CuO, as opposed to Cu2O as HIL exhibits significantly better device

performance with particularly low-driving voltage. Using ultraviolet photoelectron

spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), and in situ deposition of

NPB we obtain oxide band structure and interface dipole energy for both oxides. Hole-

injection barriers into NPB derived from UPS results agree well with transport modelling

of single carrier devices, explaining device performance differences in terms of the hole-

injection ability of each material.

3.2 Experimental

OLED devices were fabricated on glass substrates cleaned in acetone, methanol

and deionized water, followed by 15 minute UV-ozone treatment. Reference devices

were fabricated on ITO coated glass (~15 Ω/) cleaned in the same manner. Hole-only

devices were fabricated on Si(100) with 2000 nm furnace oxides for electrical isolation.

Metal and organic films were thermally evaporated in separate chambers with base

pressures of ~10-7 Torr and ~10-8 Torr, respectively. Film thicknesses were monitored

using calibrated quartz crystal monitors. As will be shown, CuO and Cu2O oxide layers

were produced by ex situ oxidation of freshly deposited Cu films for 60 minutes by UV-

ozone treatment and annealing in air at 100 °C, respectively.

Immediately after oxidation, organic and cathode layers were deposited to obtain

the following OLED structure: Anode/NPB(60 nm)/tris-(8-hydroxyquinoline)

aluminum(Alq3)(45 nm)/LiF (1.5 nm)/Al (100 nm). The structure of the hole-only

devices was as follows: Anode / NPB(500 nm)/Cu(20 nm). The active area for all

devices was 1 mm × 2 mm. Luminance and current density (L-J-V) characteristics were

29

measured in ambient atmosphere using an HP4140B pA meter and a Minolta LS-110

Luminance meter. The perpendicular anode and cathode lines were contacted

approximately 2 cm away from device active area. XPS and UPS measurements were

performed using a PHI 5500 system with monochromated Al Kα x-rays (1486.6 eV) and

He I UV (21.2 eV). All spectra were calibrated by referencing to the Fermi edge of

sputtered Au.

3.3 Results and Discussion

Figure 1 compares current density and luminance for OLEDs made with

semitransparent Cu/CuO and ITO anodes. Cu film thickness prior to oxidation was

optimized at 20 nm, compromising between optical transparency and series sheet

resistance. The Cu/CuO anode device exhibited approximately equal driving voltage as

the ITO baseline device, despite lower transparency and larger sheet resistance of the

Cu/CuO anode (25 Ω/ as opposed to 15 Ω/ for ITO). The driving voltage at 8000

cd/m2 luminance was 7.4 V for the Cu/CuO device compared with 7.6 V for the ITO

reference device. Past 7 V, the Cu/CuO device overtakes the ITO device in both current

density and luminance.

30

Figure 3.1. Current density vs. voltage and luminance vs. voltage for OLEDs made with UV-oxidized Cu and ITO anodes.

Previous reports of Cu oxide anodes6 or anode modification layers5 have not

distinguished any preferred Cu oxidation state. This distinction can be made, however,

by examining both 2p3/2 XPS and LMM Auger spectra. 9 XPS data are shown in figure

2a. The UV-ozone sample exhibits a satellite and main peak characteristic of CuO. The

heated sample has no satellite and a main peak consistent with either metallic Cu or

Cu2O. The Auger LMM peak position (Fig. 2b), however, confirms the presence of only

Cu2O.

31

The thicknesses of UV-ozone oxide was found to plateau at ~6 nm after 60 minutes of

UV-ozone treatment, as determined by XPS sputter profiling and stylus profilometry of

the sputter crater.

Figure 3.2. a) Cu2p3/2 XPS and b) CuLMM Auger spectra (hν=1486.6 eV) for 5 nm

Cu films heated at 100 °C for 60 minutes and UV-ozone treated for 60 minutes.

32

In order to elucidate the hole injecting properties of CuO and Cu2O, single-carrier

devices were fabricated. Figure 3 shows the room temperature J-V characteristics of

hole-only devices made with Cu/CuO and Cu/Cu2O anode structures. Qualitatively, it is

seen that the Cu/CuO anode exhibits a much larger hole-current. A useful model

approximating charge injection into organic materials has been described by Scott and

Malliaras,10,11 and has been used to fit the single-carrier device data. In this model,

charge recombination at the organic interface due to carriers hopping within an image

potential is used in combination with the principle of detailed balance to calculate the

electric field dependence of thermionic injection. The field and temperature dependence

of the injection current is given by:

( )2 1 204 exp b bJ N E k T fψ μ φ= − (1)

where, f is the reduced electric field,

3 2 24 bf e E k Tπε= (2)

and

( ) ( )1 21 1 2 1 1 21 2f f f f fψ − − −= + − + (3)

In the above equations, E is the applied electric field, μ is the field dependent carrier

mobility, N0 is the density of chargeable states in the organic film, bφ is the barrier

height, kb is the Boltzmann constant, T is the Kelvin temperature, and ε is the dielectric

constant of the organic material.

33

Figure 3.3. a) Current density vs. voltage for hole-only structures as follows: Anode/NPB (500nm)/Cu. The anodes used were Cu/CuO and Cu/Cu2O. Fits to the Scott and Malliaras injection model are included as solid lines, along with

bφ values, see Ref. 10 and 11. b) Current density vs. voltage and luminance vs. voltage (inset) for OLEDs made with CuO and Cu2O capped ITO anodes.

The field dependent mobility and relative permittivity for NPB deposited in our

vacuum system was obtained through time-of-flight and capacitance-voltage

measurements reported elsewhere12. The density of chargeable states in the highest

occupied molecular orbital (HOMO) (N0 = 1021 cm-3) is taken by assuming one state per

molecule. Figure 3a shows fits to the single carrier device data. Barrier heights bφ = 0.3

eV for the CuO/NPB interface and bφ = 0.8 eV for the Cu2O/NPB interface were

extracted. Figure 3b shows J-V and L-V characteristics for OLEDs made using ITO

34

modified with 5 nm of either oxide of Cu. As expected from the single carrier devices,

capping with Cu2O resulted in greatly reduced current density and luminance when

compared with CuO; this can be attributed to the reduction in hole-injection barrier

height.

X-ray and ultraviolet photoemission spectroscopies were used to investigate the

electronic structure of the oxide/NPB interface for both types of oxides. This

combination of techniques allows one to distinguish between interfacial dipole formation

and band bending, in addition to possible charging.13, 14 Films of NPB were deposited on

the oxides by step-by-step deposition, while obtaining XPS and UPS spectra in situ after

each deposition step. The final NPB thickness used for analysis was 12.8 nm, as sample

charging became significant after 25.6 nm, as seen by large XPS peak shifts when

switching the UPS photon flux on and off. Figure 4a and 4b show He I valence band

spectra for the NPB deposition sequence on CuO and Cu2O substrates. Linear fits of the

substrate valence band features and backgrounds are used to determine the valence band

maximum (VBM) positions relative to the Fermi level (Ef). These relative positions,

denoted vEΔ , are 0.1 eV in the case of CuO, and 0.3 eV in the case of Cu2O. With

increasing NBP thickness, NPB’s HOMO spectral feature becomes apparent. The

relevant hole injection barrier is the energy difference between the oxide VBM and the

HOMO maximum (HM) before band bending. HM is taken as the low binding energy

cutoff of the HOMO feature.13,14,15 Referring to the inset of figure 3a, we have,

b HM b vE V Eφ = − −Δ

35

where HME is the position of HM relative to Ef, and Vb is the degree of band bending. At

12.8 nm NPB thickness, EHM is 0.9 eV for the CuO system, and 1.1 eV for the Cu2O

system.

Figure 3.4. He I UPS valence band spectra for a) CuO and b) Cu2O substrates with

sequential NPB deposition up to a thickness of 12.8 nm. Work function shifts and C1s core level shifts vs. NPB thickness are plotted for c) CuO substrate and d) Cu2O substrate.

In order to determine the amount of band bending in the NPB, corresponding

shifts in the C1s XPS peaks were monitored for both oxide systems. In addition, work

function shifts were measured via UPS secondary cutoff. Figures 4c and 4d show that at

an NPB thickness of 12.8 nm the amount of band bending was 0.4 eV and 0.1 eV for the

36

CuO and Cu2O systems, respectively. The work function of the CuO substrate was

measured to be 5.3 eV, and that of the Cu2O substrate was 4.6 eV, agreeing with

literature values16,17. Additionally, deviation of the C1s shift from the UPS work function

shift at low NPB coverage for both material systems indicates the presence of an

interface dipole, Δ .13 The values of Δ are found to be 0.4 eV and 0.2 eV for the CuO

and Cu2O systems. Using Eq. 4, hole-injection barriers are calculated to be 0.4 eV for the

CuO/NPB interface, and 0.7 eV for the Cu2O/NPB interface, in good agreement with the

previous transport calculations.

3.4 Conclusion

In summary, it is found that UV-ozone treated Cu acts as a suitable anode for

organic light emitting diodes, imparting low driving voltage and high luminous output

despite its moderate transparency. Low cost, high conductivity, easy patternability and

low thermal budget make such ozone treated Cu-based anodes attractive for use in

commercial applications. Moreover, we have identified CuO as the oxidation state of Cu

most suited for hole-injection into NPB. Transport modelling of single carrier devices

yields calculated hole-injection barrier heights which harmonize well with the observed

electronic structure investigated via photoemission, showing a significantly smaller

barrier at the CuO/NPB interface. Although this study has focused exclusively on hole-

injection into NPB, the Cu/CuO system is a potential candidate for low junction

resistance anode contacts for other organic materials.

37

Chapter 4. Aluminum doped zinc oxide for organic photovoltaics

4.1 Introduction

Aluminum doped zinc oxide (AZO) was grown via magnetron sputtering as a

low-cost alternative to indium tin oxide (ITO) for organic photovoltaics (OPV). Post

deposition ozone treatment was found to play a critical role in enabling the use of AZO

for Cu-phthalocyanine/C60 cells. Electrical conductivity of AZO increased upon exposure

to ozone. Ozone treatment of AZO films resulted in devices with lower series resistance,

increased open-circuit voltage, and power conversion efficiency double that of devices

fabricated on untreated AZO. These improvements are explained through band

alignment. Furthermore, cells fabricated using ozone treated AZO and standard ITO

displayed comparable performance.

Organic photovoltaics (OPVs) have drawn increasing interest as a route to capture

sunlight for low-cost renewable energy. Since Tang1 introduced the first donor-acceptor

(DA) device more than a decade ago, power conversion efficiencies of OPVs have

steadily risen, reaching efficiencies higher than 5 % for small-molecule devices.2

Polymer-based photovoltaics show similar performance gains.3 All photovoltaics,

including OPVs, require the use of transparent conducting electrode materials in order to

admit light into the active region as well as to collect charge. The cost of transparent

conductive films is thus a critical component of the total OPV price. ITO is currently the

de facto standard transparent conductive oxide (TCO) used in OPV technology due its

high optical transparency, metallic conductivity, and high work function. Relatively little

investigation has occurred involving the use of alternative TCOs for small-molecule OPV

38

applications. This work is essential, however, given that ITO has become approximately

ten times more expensive over the past 5 years due to diminishing indium resources;4 the

cost of indium is only expected to grow with increasing demand from producers of

photovoltaics on top of the existing flat-panel display industry.

The properties of ZnO doped with group three elements such as Al or Ga have

been extensively investigated for use as low-cost TCOs. ZnO is a wide bandgap (3.3 eV),

n-type semiconductor that can be produced by methods including pulsed laser deposition

(PLD), chemical vapour deposition (CVD), spray pyrolysis, and magnetron sputtering.5

ZnO doped with aluminum (AZO) is well suited to PV applications owing to its high

electrical conductivity and excellent optical transparency in the near infrared and visible

regions. ITO-equivalent performance for phthalocyanine-based OPV devices fabricated

on ZnO without the need for a buffer layer has not been demonstrated.4, 6 One report of

ITO-equivalent performance, however, has been made for oligothiphene-based cells.7 In

this work, we show the importance of sputtering conditions and post-deposition ozone

treatment of AZO prior to OPV fabrication. Furthermore, we show that ozone treated

AZO is an effective ITO replacement for double-heterojunction phthalocyanine-based

devices.

4.2 Experimental

OPV devices were fabricated on glass substrates cleaned in acetone, methanol and

deionized water, followed by 15 minute ultraviolet (UV)-ozone treatment. Reference

devices were fabricated on ITO coated glass (15 Ω/) cleaned in the same manner. AZO

films were produced using dc magnetron sputtering in a BOC Edwards Auto 306 coating

39

system equipped with an MDX 1.5 K DC Plasma Generator Power Supply. The base

pressure was ~10-7 Torr. A 3-inch diameter ZnO target doped with 2 wt.% Al2O3 was

sputtered at 50 W in a 5 mTorr Ar atmosphere. Substrates were placed at 8, 12, 16 and 19

cm centre-to-centre distance from the target. The substrates were oriented perpendicular

to the target, and were offset horizontally from the target center by 6 cm. Anode lines

were defined by photolithography using Shipley S1813 photoresist and 2% HCl etchant

solution. Sheet resistance measurements of all films were taken using a Keithley 2400

SourceMeter and four-point probes. Film thicknesses were measured using a KLA

Tencor P-16+ stylus profilometer. Optical transmission and reflection spectra were taken

using a Cary spectrophotometer equipped with an integrating sphere. Surface

morphology was observed via scanning electron microscopy (SEM) and atomic force

microscopy (AFM) in tapping mode. AZO films were treated by ex situ oxidation for 60

min by UV-ozone treatment.

Metal and organic films were thermally evaporated in separate chambers with

base pressures of ~10-7 Torr and ~10-8 Torr, respectively. Film thicknesses were

monitored using calibrated quartz crystal monitors. Organic solar cells were fabricated

using copper phthalocyanine (CuPc) and Buckminsterfullerene (C60) as the donor-

acceptor (DA) system. The cell was finished using an exciton blocking layer of 1,3,5-

tris(2-N-phenylbenzimidazolyl) benzene (TPBI) and an aluminum cathode. The final

device structure was as follows: Anode/CuPc (30 nm)/C60 (30 nm)/ TPBI(6 nm)/Al (80

nm). The active area was 1 mm x 2 mm for all devices. Device current density versus

voltage (J-V) characteristics were measured using a Keithley 6430 sub-femptoamp meter

under 100 mW/cm2 simulated AM1.5G solar illumination.

40

4.3 Results and Discussion

Figure 1a shows optical transmission spectra for untreated AZO films deposited at

target-to-substrate distances of 8, 12, 16 and 19 cm. Figure 1b shows electrical resistivity

of AZO films as a function of target-to-substrate distance for as-deposited and ozone-

treated samples. Film thicknesses were 98, 164, 248, and 541 nm respectively owning to

varying deposition rates within one deposition. These thicknesses were accounted for in

the calculation of bulk resistivity. High transparency is observed throughout the visible

spectrum, with an average transmission of 92% between 380 and 750 nm for the film

deposited at 8 cm. With decreasing target-to-substrate distance, the absorption edge of

the AZO films moves towards lower wavelengths. We therefore infer by the Drude

model5 that shorter target-to-substrate distance resulted in higher free carrier

concentration for AZO films deposited in our system.

41

Figure 4.1. a) Optical Transmission vs. Wavelenth for untreated, DC sputtered AZO films deposited at 8, 12, 16 and 19 cm target-to-substrate distances. b) Film resistivity for as-deposited and ozone treated AZO vs. target-to-substrate distance.

Film resistivity increased monotonically with increasing target-to-substrate

distance, as expected from optical measurements of carrier concentration. Significantly,

42

ozone treatment lowered the resistivities of the most conductive AZO films. The

resistivity of the film deposited at 8 cm was decreased by 40 % from 2.3x10-3 to 1.4x10-3

Ωcm. At a distance of 19 cm from the target, the resistivity of as-deposited films

increased abruptly by approximately three orders of magnitude. Interestingly, these films

became much more resistive after ozone treatment. SEM images of as-deposited AZO

films are shown in figure 2. With increasing target-to-substrate distance, grain sizes

became progresssively smaller. A grain size of approximately 50 nm was observed for

the most conductive film deposited at 8 cm. The surface roughness of this film was found

to be 6 nm by AFM. Increased conductivity at shorter range is attributable to increased

carrier mobility (as expected from larger grain size and decreased grain boundary

scattering), in addition to the higher carrier concentration observed optically. Since

doping in AZO is thought to arise from Al subsitition on Zn cationic sites,8 we posit that

the increase in conductivity with UV-ozone treatment for the most conductive films was

due to a reduction in the number of oxygen vacancies in the AZO lattice, leading to

reduced impurity scattering and higher electron mobility. No observable change in grain

structure was seen by SEM after uv-ozone treatment. Lower carrier concentration with

target-to-substrate distance indicates less Al incorporation into the ZnO lattice and

subsequent segregation to grain boundaries or incorporation into Al2O3. For the least

conductive film, the observed increase in resistivity with UV-ozone treatment was likely

due to the reaction of segregated aluminum with oxygen, producing insulating Al2O3.

43

Figure 4.2. SEM micrographs of AZO films grown at target-to-substrate distances of a) 19 cm, b) 16 cm, c) 8 cm.

Double heterostructure cells were prepared on the highest conductivity as-

deposited (device A) and uv-ozone treated (device B) AZO films in a single deposition

sequence. The sheet resistances of these films were 43 and 26 Ω/, respectively. Figure

3a shows the current-voltage characteristics of both types of device under 100 mW/cm2

simulated solar illumination. A noticeable change in device performance occured

following ozone treatment. Cell B exhibited higher open circuit voltage ( OCV ), and lower

series resistance. A power conversion efficiency of 1.3% was attained for device B as

compared to 0.7% for A. The forward bias dark current of sample B was increased and

44

the reverse bias current was decreased, indicating a reduced hole injection barrier and a

larger barrier to electron injection for ozone treated AZO into CuPc. XPS secondary

cutoff mesurements indicate an increase in work function from 4.2 eV as deposited to 4.7

eV after in situ ozone treatment of unpatterned AZO.

Figure 4.3. a) Current density and b) Photocurrent density vs. voltage for OPV devices made using as-deposited and ozone treated AZO. Current densities were measured under 100 mW/cm2 and dark conditions. Photocurrent density at a specified voltage was calculated as the difference between device current densities measured under illuminated and dark conditions.

45

Figure 3b shows photocurrent-voltage curves for devices A and B obtained by

subtracting dark from light current densities. Ozone treatment resulted in a clear shift of

the photocurrent characteristic toward higher bias ( 0.2V VΔ = ). When taken with the

aformentioned increase in forward bias dark current, this shift indicates an improved

band alignment for ozone treated AZO, bringing the AZO Fermi level 0.2 eV closer to

CuPc’s highest occupied molecular orbital (HOMO) upon contact.9 Consequently, device

B has a larger built-in voltage ( biV ). Figure 3c schematically shows the energy level

alignment for samples A and B under open circuit conditions. Using the Einstein

relation,10 the sum of drift and diffussion currents for electrons can be written in terms of

the gradient in quasi-Fermi level, FnE∇ :

( ) ( ) ( )n n FnJ x en x E xμ= ∇ (1)

where e is the charge of an electron, ( )n x is the carrier concentration, and nμ is the

electron mobility.11 A similar relation applies for holes. At OCV , zero net current flows as

drift balances diffusion and the quasi Fermi levels FnE and FpE are constant from the DA

interface to the contacts as shown in figure 3c. Furthermore, assuming no voltage drop

across the contact interfaces, OCV equals the maximum splitting of the quasi Fermi levels

across the solar cell.11

OC Fn FpV E E= − (2)

46

The amount of splitting rises as the free carrier concentration in each layer rises. The

electric field developed across the organics due to quasi Fermi level splitting under

illumination counteracts the built-in field. The net potential gradient across the organics

is thus the difference between OCV and biV .12 This net potential gradient, which tends to

confine free carriers to the DA interface, must be smaller in device B due to its larger biV ,

as shown in figure 3c. Less interface recombination due to reduced carrier accumulation

at the DA interface, and improved exciton dissociation are therefore expected for device

B. The resulting increase in free carrier concentration in device B leads to higher

photocurrent close OCV , as well as an increase in OCV itself due to greater quasi Fermi-

level splitting.12

A sigmoidal J-V characteristic was observed for device A, but not for device B.

Approaching 0.4 V, the electric field in the organics begins to oppose the flow of

photogenerated free carriers, causing carrier accumulation at the at the DA interface. As

mentioned, this effect is more pronounced for device A due its smaller built-in potential.

Higher carrier concentration at the DA interface increases recombination and lowers the

observed photocurrent. Concurrently, holes are being injected into CuPc from the AZO

electrode. Due to a larger hole injection barrier for device A, the forward current remains

low. As the forward bias is further increased, holes begin to overcome the injection

barrier, driving a forward current to complete the sigmoidal JV characteristic.

In order to demonstrate ozone-treated AZO as an ITO-alternative, devices

employing AZO (device C) and ITO (device D) anodes were fabricated in a single

deposition sequence. Figure 4 shows J-V charactreristics of devices C and D.

47

Approximately equivalent performance was achieved by devices C and D, with power

conversion efficiencies matching within 10%. The measured short circuit currents were

nearly identical, from which we may conclude that there was little change in the optical

field distribution, absorption of light in the active region, or in charge collection

efficiency. Interestingly, devices made with AZO exhibited higher open circuit voltages

than did those made with ITO. Table 1 compares relevent performance parameters for all

devices. Reciprocal slopes of J-V relations at OCV are used used to compare the series

resistances, SR ,_of_the_devices.13

Figure. 4.4. Current density at 100 mW/cm2 and dark illumination levels vs. voltage for OPV cells made using ozone treated AZO as well as ITO.

48

Table 4.1. Open circuit voltage VOC, short circuit current density JSC, fill factor FF, Power Conversion Efficiency ηP, and the calculated series resistance RS of all cells. Experiment Device Anode VOC

(V)

JSC

(mA/cm2)

FF ηP

(%)

RS

(Ωcm2)

1 A AZO 0.42 4.46 0.37 0.7 65

B AZO (O3) 0.50 4.53 0.55 1.3 19

2 C AZO (O3) 0.49 3.96 0.53 1.0 24

D ITO 0.46 4.04 0.60 1.1 18

4.4 Conclusion

In sum, we have shown that ozone treated AZO is an effective ITO alternative

material for CuPc-based organic photovoltaics. The optical transparency and conductivity

of AZO was found to vary considerably with target-to-substrate distance. Optical

transmission of >90% and resistivity of 1.4x10-3 Ωcm were achieved. Treating AZO

films with UV-generated ozone resulted in improved conductivity. OPV cells fabricated

on ozone treated AZO outperformed cells fabricated on as-deposited AZO, showing

higher OCV , fill factor, and power conversion efficiency. These improvements were

explained through band alignment under OCV conditions employing quasi Fermi-level

splitting. Finally, devices prepared with ozone treated AZO and ITO anodes were

compared. Equvalent performance was obtained, showing low series resistance, high fill

factor and power conversion efficiencies matching within 10%.

49

Chapter 5. Transparent Conductive Oxides for Organic Photovoltaics

5.1 Introduction

Fluorine doped tin oxide (FTO) and aluminum doped zinc oxide (AZO) were

systematically investigated as alternatives to indium tin oxide (ITO) for canonical poly(3-

hexylthiophene) (P3HT) + [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) polymer

bulk heterojunction (BHJ) solar cells. Devices made with FTO performed twice as well

as devices made with ITO, establishing FTO as a suitable, low-cost ITO replacement.

Ozone treatment was shown to be a critical enabling element for both FTO and AZO. X-

ray photoelectron spectroscopy (XPS) and device characteristics were used to explain the

effect of ozone treatment and the origin of open circuit voltage.

Successful development of efficient, low cost organic photovoltaic devices

(OPVs) will have sweeping implications for worldwide energy production. Since Tang

first introduced organic donor-acceptor OPV devices over a decade ago,1 power

conversion efficiencies have risen above 5% for both polymer and small molecule based

devices.2 Lifetimes of more than 10,000 hours have also been recently demonstrated.3

The use of sprayable or printable photoactive polymer materials offers inherent cost

savings needed for solar energy adoption in the competitive marketplace. In addition to

organic materials, transparent conductors comprise one of the major cost drivers for

organic solar cells. Indium Tin oxide (ITO) remains the de facto standard transparent

conducting oxide (TCO) used in OPV cells due to its combination of high optical

transparency, high work function, and metallic conductivity.4 The cost of ITO, however,

50

is expected to increase substantially due to the rarity of Indium base metal combined

with surging market demand arising from flat panel displays.

In this work, fluorine tin oxide (FTO) and aluminum doped zinc oxide (AZO) are

systematically examined as ITO-replacement materials for canonical poly(3-

hexylthiophene) (P3HT) + [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) polymer

bulk heterojunction (BHJ) solar cells. TCO replacement materials have been identified as

an area of strategic research; 5 nevertheless, relatively few studies of alternative TCO

materials for OPVs have been published to date.6-10

Both FTO and AZO possess high electrical conductivity and optical transparency.

FTO can be produced cheaply via spray pyrolysis and is typically used for dye sensitized

solar cells.11 AZO can be produced by various means, including DC sputtering, and

requires no high temperature annealing.12 We demonstrate a doubling of OPV

performance with the use of ozone treated FTO as opposed to a standard ITO anode

without the need for a buffer layer. We identify ozone surface treatment as an essential

enabling element for both FTO and AZO anodes, and explain the effect of ozone

treatment using x-ray photoelectron spectroscopy (XPS). Finally, an explanation for the

origin of open circuit voltage is proposed based on OPV device characteristics and XPS

work function measurements.

51

5.2 Experimental

OPV devices were fabricated on glass substrates cleaned in acetone, methanol and

deionized water, followed by 15 minute ultraviolet (uv)-ozone treatment. Reference

devices were fabricated on ITO coated glass substrates (15 Ω/) purchased from

Colorado Concept Coatings Inc. and cleaned in the same manner. AZO films (90 Ω/)

were produced using dc magnetron sputtering in a BOC Edwards Auto 306 coating

system equipped with an MDX 1.5 K DC Plasma Generator Power Supply. The base

pressure was ~10-7 Torr. A 3-inch diameter ZnO target doped with 2 wt.% Al2O3 was

sputtered at 50 W in a 5 mTorr Ar atmosphere. FTO coated glass (25 Ω/) purchased

from Solaronix Inc., was used following cleaning as per above. Sheet resistance

measurements of all films were taken using a Keithley 2400 SourceMeter and four-point

probes. Film thicknesses were measured using a KLA Tencor P-16+ stylus profilometer.

Optical transmission spectra were taken using a Cary spectrophotometer. Surface

morphology was observed via scanning electron microscopy (SEM) and atomic force

microscopy (AFM) in tapping mode. XPS measurements were performed using a PHI

5500 system with monochromated Al Kα x-rays (1486.6 eV). All spectra were calibrated

by referencing to the Fermi edge of sputtered Au. ITO, AZO, and FTO films were

treated by ex situ uv-ozone for 60 min.

PCBM (17mg/mL):P3HT(17mg/mL) solution was prepared in 1,2-

Dichlorobenzene in a nitrogen-filled glove box, then stirred in dark for 8 hours at 50.

This solution was spin-coated on the various TCO substrates at 500rpm for 9s, then at

1100rpm for 45s. The polymer layer was dried for 24 hours at room temperature in a

52

glove box, and the resulting film thickness was found to be 65 nm. Lithium fluoride and

aluminum films were thermally evaporated in a vacuum chamber with base pressure ~10-

7 Torr. Circular aluminum cathodes were used to define a device active area of 4.45 mm2

in all cases. Film thicknesses were monitored using a calibrated quartz crystal monitor.

The final device structure was as follows: Anode/ PCBM:P3HT (65 nm)/ LiF (1 nm) )/Al

(80 nm). Device current density versus voltage (J-V) characteristics were measured

using a Keithley 6430 sub-femptoamp meter under 100 mW/cm2 (1 sun) simulated

AM1.5G solar illumination.

5.3 Results and Discussion

As will be shown below, ozone treatment of FTO and AZO alternatives is

essential for efficient BHJ device performance. It is well known that treatment of ITO

with ozone serves to increase its work function and improve the performance of devices

made therewith. Importantly, we find that a similar increase in work function results for

both FTO and AZO films upon treatment. Figure 1a shows the XPS secondary cutoffs for

treated and untreated AZO and FTO. In both cases, ozone treatment was found to

increase the work function of the TCO by 0.4 eV, resulting in a 0.4 eV shift in the

secondary cutoffs towards lower binding energy. A common explanation for this increase

in work function is band bending due to carrier accumulation at the oxide surface13. Band

bending would cause both a core level shift and a change in work function.14,15 Figure 1b

shows the Sn 3d5/2 and Zn 2p3/2 spectra for ozone treated and as-deposited FTO and AZO.

No appreciable core level shifts are observed after ozone treatment. Therefore, band

bending alone does not explain the increases in work function. We posit the formation of

53

dipole layers arising from oxygen termination of the TCO surfaces during ozone

treatment as the cause of the work function increase. Such dipole layers would raise the

apparent material work function without creating a depletion region that would cause a

core level shift.14,15

Figure 5.1. Secondary cutoffs (a) and core level shifts (b) for ozone treated and untreated AZO and FTO.

Figure 2a shows the JV characteristics of BHJ cells made with ozone treated FTO,

AZO and ITO measured under 1 sun illumination. Significantly, the FTO device achieved

a power conversion efficiency almost twice that of the other cells. Ozone treated FTO is

54

therefore suitable as an inexpensive and superior anode replacement material for

PCBM:P3HT based devices without the need for a buffer layer. Table I shows the

relevant parameters for devices shown in figure 3a.The FTO device exhibited much

higher shunt resistance, shuntR , and short circuit current, SCJ , than the other devices. A

low shuntR is detrimental, and can arise from the leakage of photogenerated electrons to

the anode during device operation; this leakage current subsequently reduces SCJ as

electrons and holes recombine at the anode. We therefore conclude that ozone treated

FTO provides a more suitable barrier to photogenerated electrons when compared to

AZO and ITO.

TABLE 5.1. Open circuit voltage VOC, short circuit current density JSC, fill factor FF, Power Conversion Efficiency ηP, calculated series resistance RS, and shunt resistance Rshunt of cells fabricated on various transparent conductive anodes. Device Anode VOC

(V) JSC

(mA/cm2)

FF ηP (%)

RS

(Ωcm2)

RShunt

(Ωcm2)

A FTO 0.55 9.45 0.54 2.79 2.26 1031 B C

AZO ITO

0.45 0.55

5.60 5.45

0.49 0.47

1.24 1.40

3.47 8.21

257 299

Figure 2b shows device JV -characteristics for both ozone treated and untreated

samples. Prior to ozone treatment, AZO acted as an Ohmic contact for electrons to the

polymer blend. In this case, the cell behaved as an electron-only device. The untreated

FTO device exhibited poor short circuit current and open circuit voltage, OCV . Ozone

treatment is therefore essential to device performance when using alternative TCO anode

materials.

55

Figure 5.2. a) JV curves for PCBM:P3HT solar cells under 100 mW/cm2 AM1.5G illumination and dark (inset). b) JV curves for PCBM:P3HT devices fabricated on untreated and ozone treated FTO and AZO under 100 mW/cm2 AM1.5G illumination

The present study of alternative transparent anodes with dramatically different

work functions offers a perspective into the origin of open circuit voltage in BHJ devices.

Consider an idealized BHJ device with a photoactive region sandwiched between two

barrier-free contacts. Within such an idealized device, electron current can be calculated

from the gradient in quasi Fermi level, FnE∇ , using the Einstein relation.

( ) ( ) ( )n DRIFT DIFFUSION n FnJ x J J en x E xμ= + = ∇ (1)

56

where e is the charge of an electron, ( )n x is the carrier concentration, and nμ is the

electron mobility.16 A similar relation applies for holes. At OCV , current flow is zero, and

the quasi Fermi levels FnE and FpE are constant. Assuming no voltage drops at the contact

interfaces, OCV equals the maximum splitting of the quasi Fermi levels across the solar

cell.16

OC Fn FpV E E= − (2)

Due to the homogeneity of the active region in a bulk heterojunction, the

concentration of charges being generated is approximately uniform throughout the

photoactive layer. Although concentration gradients may exist locally in PCBM and

P3HT phases, no significant concentration gradients of photogenerated charge are

expected over the entire thickness of the photoactive region. Consequently, we expect

the device current to be dominated by the drift current.

( )DRIFT n pJ e n p Eμ μ= + (3)

At the OCV condition, zero net current flows. Therefore, we have that DRIFTJ is

approximately nil, implying zero field across the device according to equation 3. Figure

3a shows the potential difference developed across a BHJ device due to quasi Fermi level

splitting under illumination. For zero field and zero drift current, we must have that

0bi OCV V− ≈ (4)

57

Any further quasi Fermi level splitting will introduce a nonzero electric field, yielding a

nonzero drift current. Therefore, the open circuit voltage cannot exceed the built-in

potential significantly in this idealized situation.

Figure 5.3. a) Schematic band alignment of a bulk heterojunction device under the condition VOC < Vbi. b) Band alignment of a BHJ device with an electron blocking anode under the condition VOC > Vbi.

Turning to our devices, we see that the device made with ozone treated FTO

exhibited a OCV larger than biV calculated from the work function difference of the FTO

and Al contacts. Furthermore, the FTO device showed higher OCV than the device made

with AZO, despite having a lower built-in potential. The reason for this apparent

discrepancy lies in the ability of the various TCOs to block electrons from exiting the

active region, which was not taken into account in the discussion above. As the forward

58

bias is increased beyond biV , photogenerated electrons will tend to move towards the

anode. If there were no barrier to electron flow out of the device, a forward current would

result, and the open circuit condition would no longer exist. However, if a barrier to

electron flow is imposed at the anode, electrons will tend to accumulate at the anode

contact. This scenario is depicted in figure 3b. An entropic diffusion current to oppose the

drift current will thus develop, allowing a zero current condition at higher bias. Thus, an

anode contact which tends to block electron flow can cause OCV to exceed biV . Devices

which possess high biV are capable of developing larger OCV despite a lack of electron

blocking ability at the anode. In the case of the ITO, for instance, a reasonable open

circuit voltage was still achieved despite a low shunt resistance. On the other hand, the

FTO device showed an open circuit voltage of 0.55 V despite having a built in potential

of only 0.3 V, owing to the anode’s ability to block electrons, as evidenced by the high

device shunt resistance.

5.4 Conclusion

In conclusion, we have shown that FTO can be used as an indium-free TCO

alternative to ITO for PCBM:P3HT devices, yielding power conversion efficiencies twice

those of devices fabricated on ITO, without the need for a buffer layer. Using XPS core

level and secondary spectra, it was shown that ozone treatment increased the work

function of FTO and AZO without causing core level shifts. The increase in work

function was consequently ascribed to the formation of a dipole layer due to oxygen

termination of the surface. Finally, using device characteristics in conjunction with XPS

59

surface measurements, the increase in efficiency observed for the FTO anode, and the

open circuit voltage achieved by the various devices, was explained using a classical

quasi-Fermi level framework.

60

Chapter 6. FTO for PbS Colloidal Quantum Dot Photovoltaics

6.1 Introduction

Solution processable materials for photovoltaics offer a low-cost approach to

producing large area solar installations. Polymer OPV systems have received much

attention in recent years due to the fact that photoactive polymer layers can be spin

coated, spray coated, or inkjet printed with relative ease. Small molecule and π-

conjugated polymer materials used in organic photovoltaics typically absorb in the visible

regime, leaving a large fraction of the infrared untapped. Conjugated polymers have

generally shown spectral sensitivity only to 1000 nm [1]. Fully half of the sun’s spectral

power lies beyond 700 nm in the infrared [2]. There is therefore a need to extend the

spectral reach of absorber materials to capture infrared light, which necessitates

investigation of alternative material systems.

One promising approach to harvesting light in the NIR spectral range is the use of

colloidal quantum dots (CQDs). Semiconducting CQDs, typically nanocrystals made

from material combinations of II-VI semiconductors such as PbS, CdS, or CdTe, offer the

possibility of tuneable optical absorption extending into the near infrared. The bandgap of

these materials can be increased or decreased through quantum size effect by controlling

the average diameter of nanocrystals produced during synthesis [3]. Colloidal quantum

dots can be synthesized in solution, employing controlled nucleation and growth with the

help of ligands such as oleic acid [4]. Because of the necessity of employing organic

ligands, photovoltaic CQD cells can be considered organic-inorganic hybrids. Often,

61

nanocrystals are introduced into polymer matrices to sensitize PV devices to a certain

spectral range [5]. Recently, CQD cells have achieved monochromatic power efficiencies

of over 4% [6]. The use of stacked CQD absorber layers, with each layer tuned to a

different spectral absorbance, is a potential method for capturing more incoming sunlight.

Fabrication of CQD cells in tandem with polymer or small molecule cells may offer a

further route to capturing light across the sun’s entire spectral range.

The most efficient CQD cells produced to date employ PbS quantum dots in

simple planar structures as follows: ITO/PbS CQD film/Al [7]. In this structure, the PbS

films forms a Schottky junction with the aluminum contact. The relative permittivities of

PBS CQD films, measured to be 17 + 2, are larger than for organic films [8]. Therefore,

excitons generated in CQD films upon photon absorption tend to be much more weakly

bound than for corresponding organic films. The carrier diffusion lengths for PbS films

have been estimated as 100 nm [8]. Unfortunately, the optical absorption length for PbS

CQD films is approximately 1000 nm, mandating films of this same depth to achieve

efficient light capture [8]. Carriers generated at a distance greater than one diffusion

length (100 nm) from the PbS/Metal schottky junction, however, will tend to recombine

before reaching the junction. A corresponding bottleneck exists for organic films, whose

optical absorption length is typically 100 nm and whose exciton diffusion length is

typically 5 -20 nm. A central challenge for both types of cells will therefore be to raise

diffusion lengths by raising carrier mobilities.

As with organic cells, all CQD cells require the use of a transparent conducting

electrode. To our knowledge, ITO has been used almost exclusively for nanocrystal solar

cell fabrication at the time of this writing. In one case, nanoporous ITO was used in an

62

attempt to raise the active areas for PbS cells [9]. In the Schottky structure decribed

above, the transparent anode is required to make an Ohmic contact with the CQD film for

efficient hole extraction. In the following work, fluorine doped tin oxide is shown to be a

suitable low-cost, indium-free alternative for use in PbS CQD cells.

6.2 Experimental

CQD devices were fabricated on FTO coated glass substrates purchased from

Solaronix Inc. The FTO glass was 15 Ω/, with 400 nm films applied to 3 mm sheet

glass. Reference devices were fabricated on ITO coated glass (~15 Ω/) substrates. All

substrates were cleaned in acetone, methanol and deionized water, followed by 15 minute

UV-ozone treatment. PbS nanocrystals were synthesized using an organometallic route.

The oleic acid capped nanocrystals were subsequently ligand-exchanged to n-butylamine

in a nitrogen filled glovebox as described elsewhere [4]. Following the ligand exchange,

CQD films were solution cast into films from 150 mg/mL octane solution by spin coating

at 500 rpm for 1 minute. Photovoltaic tests were performed using a Keithley 6430 sub-

femptoamp meter for IV scans, and a 975 nm laser diode providing illumination at 80

mW/cm2.

Metal films were thermally evaporated in a BOC Edwards Auto 306 coating

system with base pressure of ~10-7 Torr. Film thicknesses were monitored using a

calibrated quartz crystal monitor. Two photovoltaic device structures were fabricated

simultaneously as follows: Device A: ITO/PbS/Al; Device B: FTO/PbS/Al. Device A

was a baseline PV device made with ITO with which to compare device B made with

FTO. Two hole-only devices were also fabricated as follows: Device C: ITO/PbS/Au;

63

Device D: FTO/PbS/Au. Gold is a known Ohmic contact to PbS CQD films [10].

Therefore, the Ohmicity of the FTO vs ITO contacts to the PbS films could be observed

in these devices. The various devices are depicted in figure 6.1.

Au Au

AlAl

ITO FTO

ITO FTO

A B

C D

Figure 6.1. Photovoltaic cells (a and b) and hole only devices (c and d) employing FTO and ITO anode materials. Device structures are as follows: a) ITO/PbS/Al, b) FTO/PbS/Al, c) ITO/PbS/Au, d) FTO/PbS/Au.

FTO films are typically much rougher than ITO films of corresponding

conductivity. Figure 6.2 a) shows an AFM image of the surface of cleaned, ozone treated

FTO. The RMS roughness of the FTO film was found to be 13.1 nm. For comparison,

figure 6.2 b) shows an AFM image of a polished, cleaned, and ozone treated ITO surface.

The RMS roughness for the ITO surface is 3.9 nm. For the specific application at hand,

64

surface roughness is not problematic due to the fact that voids in the substrate surface are

filled during spin coating.

Figure 6.2. AFM image of a) FTO and b) ITO surfaces. The surface of unpolished FTO is much rougher than that of polished ITO.

6.3 Results and Discussion

Figure 6.3 shows the IV response of devices A and B under dark conditions and

under 90 mW/cm2 illumination. The open circuit voltages for both the ITO- and FTO-

based devices were the same at 0.34 V. The short circuit current for the FTO device B

was 6.84x10-5A, and that of device A was 7.65x10-5. Therefore the device performances

were comparable within 10% power conversion efficiency and can be considered

equivalent. Typical device-to-device variability in short circuit current for cells having

nominally identical structure is larger than 10%, owing to variations in the spin coating

fabrication process.

65

Figure 6.3. Current at 80 mW/cm2 and dark illumination levels vs. voltage for CQD cells made using ozone treated ITO (device a) and FTO (device b).

Figure 6.4 depicts the IV relations for devices C and D under dark conditions and

under 90 mW/cm2 illumination. Both devices show photoconductive effect under

illumination, but no photovoltaic effect. As mentioned above, Au films are known to

form Ohmic contacts with PbS nanocrystal films. The linearity of the IV relation for

device C therefore indicates that ITO is also Ohmic contact. The IV relation for device D,

on the other hand, shows a very slight sub-linearity under both dark and light conditions.

The existence of a weak Schottky barrier is expected for the FTO case due to the fact that

the work function of ozone treated FTO (4.5 eV) is lower than that of ITO (5.0 eV).

Nevertheless, the relative weakness of the barrier is inconsequential to device

performance, as demonstrated.

66

Figure 6.4. Current at 80 mW/cm2 and dark illumination levels vs. voltage for CQD hole-only devices made using ozone treated ITO (device c) and FTO (device d).

6.4 Conclusion

In conclusion, FTO has been shown as a suitable ITO-free alternative transparent

conductive anode material for PbS-based PV for the first time. Devices made with FTO

and ITO exhibited the same short circuit voltage, despite the fact that the anode materials

had work functions differing by 0.5 eV. Furthermore, CQD solar cells fabricated with

FTO and ITO displayed power conversion efficiencies equivalent within 10% EQE.

Equivalent device performance is attributed to the ability of FTO to form a nearly Ohmic

contact with PbS nanocrystals.

67

Chapter 7. Summary

In the present work, suitable ITO anode replacement materials were demonstrated

for small-molecule, polymer, and PbS colloidal quantum dot photovoltaics, as well as

OLEDS. Table 7.1 summarizes the matrix of TCOs and devices tested in the present

work. Cu/CuO anodes were not applied to solar cells due to the fact that they are not

transparent over the full visible spectrum; the peak in optical transmission was at 575 nm

for these films, however, which was close to the AlQ3 emission peak at 525 nm, allowing

for efficient green OLED devices. FTO substrates were not applied to small molecule

solar cells due to an RMS surface roughness (12 nm RMS) around half the thickness of

typical organic absorption layers (30 nm); device shorting and low shunt resistance is

expected for such devices.

Table 7.1. TCO Properties Anode FTO AZO ITO Cu/CuO Transmissivity (%)

85%

92%

95%

50% @ 575 nm

Rsheet (Ω/) 15 26-90 15 25 Φ (eV) 4.5 4.7 5.1 5.3 Relative cost $$ $$ $$$$$ $ RMS Roughness (nm) 13 6 3.9 3 Process Spray

Pyrolysis

DC Sputtering RF Sputtering

Evaporation

Device efficiency Small Molecule Bulk Heterojunction

- 2.79

1.0 1.24

1.1 1.4

Colloidal Quantum Dot OLED

-

Comments

68

Ultrathin films of copper, when treated with ozone gas to form Cu/CuO bilayers,

were found to work efficiently as ITO-free anodes for OLEDs; the high conductivity of

the copper films allowed them to be made ultrathin, which ensured a degree of optical

transparency close to the emission peak of AlQ3-based OLED devices. The high work

function and smoothness of these films provided an excellent contact for hole injection at

the OLED anode contact.

For the various solar cell applications, oxides with a high transparency over the

entire visible range were required. FTO and AZO were selected as Indium-free

replacements in these applications due to their clarity, high conductivity,

chemical/mechanical stability, and non-toxicity. AZO as opposed to FTO was applied to

small-molecule OPV devices due to its lower RMS roughness. AZO and FTO were both

applied to polymer bulk-heterojunction cells due to less stringent requirements on

roughness for this case. Small molecule cells made with ozone treated AZO were found

to perform just as well as identical devices made with ITO. Ozone treatment prior to

device fabrication was found to be a critical enabling element for the use of AZO in this

application. Polymer solar cells fabricated on FTO were shown to produce devices that

were twice as efficient as their ITO-based counterparts. Analysis of device characteristics

as well as XPS analysis were used to explain the origin of the improvements in device

performance. FTO was also shown as a suitable ITO-free alternative anode material for

PbS colloidal quantum dot photovoltaic devices. Once again, the requirements for surface

roughness were not stringent in this case due to the ability of the nanocrystals to

intercalate into voids and crevices. Analysis of hole-only devices showed that FTO

provided a quasi Ohmic contact to PbS, showing only a slight rectification at the FTO

69

PbS junction. When used as an anode material for PbS solar cells, FTO performed

similarly to ITO, resulting in devices of equivalent open circuit voltage, and efficiencies

equivalent within 10% EQE.

Although suitable ITO replacement materials were found for the various classes

of devices studied above, these remains much room for improvement in terms of further

lowering manufacturing costs. Of particular importance is the need to develop spin-

coatable, spray-coatable or printable transparent conductive materials that provide ITO-

equivalent performance when incorporated into organic devices. Investigation into such

material systems is currently underway. In particular, films of graphene, carbon

nanotubes and metallic nanowires have been applied to organic solar devices. Initial

attempts show promise, but more work is required. Graphene sheets [1] and carbon

nanotubes [2] do not match the conductivity of ITO. Metallic nanowires have shown to

be competitive with ITO in terms of transparency, conductivity and organic solar cell

performance [3]; however, the need for noble metals resulting in high costs, or the

problem of oxidation of non-noble metals resulting in decreased device lifetime, may

impede this method.

Recently, spin-coatable sol-gel synthesized antimony doped tin oxide (ATO) has

been demonstrated as a TCO [4]. Initial results from Lu group show that ATO rivals FTO

as an anode contact for PCBM:P3HT solar cells. More studies are required in order to

optimize the films to achieve higher conductivity. The production of spin-coated

conductive ATO or other oxide would break new ground if successful applied to polymer

70

or small molecule solar cells. Such material systems would enable low-cost, flexible TCO

films that are roll-to-roll processible, highly conductive, and robust.

71

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